In this article, we collect a series of laboratory experiments which
mainly concern surface phenomena and colloidal systems. Due to their number, these
experiments will be briefly described. As you know, our articles do not intend to supply
an exhaustive explanation of the topics we deal with, but rather to give rise to a
curiosity toward them and to give young people exposure to interesting categories of
natural phenomena. We try to achieve these goals through practical and experimental
activities in which hands-on involvement plays an important role. The way these activities
are presented tends to bring out their amusing aspects and, hopefully, to establish a
positive attitude towards the subjects treated. We are convinced that these activities
will arouse questions in the participants and that they themselves, through curiosity,
will go looking for explanations. We are also convinced that feeding the desire to know
the how and why of things, that is, the curiosity spawned within each student
is more important and useful than explanations supplied when a student feels no need.

As the explanations are short, we have often supplied links to Internet
websites and sometimes a bibliography. Unfortunately, we will not be able to continually
update these links, so we have supplied keywords to look for updated information on the
Internet using search engines. Do not use all these keywords together, but in the
combinations which seem more relevant to you. During these searches, often too much
documentation is found, much of which is useless to our search. So, if necessary, add a
term like these: school, students, experiment, test, classroom, homemade, homework,
science fair, science project, lesson, lesson plan.

WARNING: Some of these
experiments can be dangerous. When children are doing them, an adult must always be
present to avoid any damage or harm. In any case, we do not assume any liability.
As for the safety and the liabilities, we recommend you to read our Warning page.

Have a good time!

INTRODUCTION TO SURFACE PHENOMENAWhy do some insects succeed in skating on water instead of sinking? Why in some cases,
does the water sprinkled on a glass surface collect into drops and in other cases spread
like a thin film? Why does water climb up a thin tube? Why can you make bubbles with soapy
water and not with tap water? For reasons we will see later on, the surface of a substance
has special properties. These surface properties are what allow these strange phenomena we
have mentioned. Not only that, but the surface is also the place of contact among
different substances. In short, the properties of surfaces are so special and important
that there is a branch of science, the physics of surfaces, devoted to the study
of surface phenomena.

A molecule of a liquid attracts the molecules which surround it and in its turn it is
attracted by them (figure 2). For the molecules which are inside a liquid, the resultant
of all these forces is neutral and all them are in equilibrium by reacting with each
other. When these molecules are on the surface, they are attracted by the molecules below
and by the lateral ones, but not toward the outside. The resultant is a force directed
inside the liquid. In its turn, the cohesion among the molecules supplies a force
tangential to the surface. So, a fluid surface behaves like an elastic membrane which
wraps and compresses the below liquid. The surface tension expresses the force
with which the surface molecules attract each other. A way to see the surface tension in
action is to observe the efforts of a bug to climb out of the water. On the contrary,
other insects, like the marsh treaders and the water striders, exploit the surface tension
to skate on the water without sinking. Here are some simple experiments using surface
tension:

1 -The floating needle.

Carefully place a
needle on the surface of a glass of water. If the water does not completely wet it, you
will see the needle float. To avoid your fingers disturbing the surface as you place the
needle, you can make a small cradle from wire to hold the needle as you lower it gently on
to the surface of the water. Another way to make it easier to float an object heavier than
water using only the surface tension is to first float a strip of tissue paper and lay the
needle on it. Slowly, the water will soak the strip, which will eventually sink, while the
needle will remain on the surface.

Figure 3 - Floating needle. At the bottom of the pot you can see the sunken strip of
tissue paper.

2 - Make sulfur powder sink. Sprinkle some sulfur powder over a
glass of water (You can buy sulfur in a hardware store). Sulfur is hydrophobic enough to
float on the water. Add a drop of detergent and you will see the particles of sulfur sink.
This experiment also works with talcum powder which you probably already have in your
home.http://www.ilpi.com/genchem/demo/tension/
has a short movie on this experiment and a description of the properties of
surfactants.

3 - Launch of the needle. With some steel wire, make a ring. Place a
needle on the ring and submerge in soapy water. When you extract the ring, two membranes
will be formed: one at the left side of the needle and the other at the right side. Now,
with a finger burst one of these membranes. The needle will be thrown away by the surface
tension of the remaining membrane, which quickly contracts, in an effort to achieve the
smallest possible surface area.

4 - The strength of the soap films.

With
some iron wire, make a "U" frame and a slider, as shown by the figure 4. Plunge
the frame in soapy water. When you extract it, you will see that the slider will be drawn
toward the bottom of the frame by the surface tension of the soap membrane. By holding the
slider still with your fingers, you can feel the force of the membrane.

5 - Measuring the surface tension. In order to measure the surface
tension of a liquid, you can use an equal-arm analytical balance. As shown by the figures
5 and 6, hang a U-shaped steel wire under one of the two weighing pans (A). By lowering
the A arm and then by lifting it up again, make a membrane to form in the U-shaped frame.
Balance it with some masses on the weighing pan B. At this point, break the film. The
balance will go down by the B side, therefore restore the equilibrium placing some masses
on the side A. The value of these last masses (F) corresponds to the force with which the
membrane tends to close into the liquid. The surface tension (T) is given by the force (F)
divided by the width (W) of the membrane, divided again by two because it is necessary to
keep into account the membrane possess two surfaces. So, T = F/2W. The value of the
surface tension of the distilled water is 7,42 g/m at 20°C and that of ethyl alcohol is
2,27 g/m always at 20°C. We supply to you these values because you will be allowed to
compare with them those you obtain through experimentation. If you do not possess an
analytical balance, you can build one of them. It will not be as exact, but it will allow
you to do these measures. Given the forces which play in this experiment, the balance
should have an accuracy of a hundredth of a gram at least.http://www.pvri.com/sp/BalBuild.htm
How to build a no cost sensitive balance (by Salvatore Previtera)http://userpages.prexar.com/dwilliamsmaine/scale/scale.html
A Home-made Balance Scale (by Dan Williams)

6 - Other method to measure the surface tension. To measure the
surface tension of liquids, you can use a metal wire ring of the diameter comprised
between 3 and 4 cm, instead of the "U" frame we have described. This wire should
be made of platinum, anyway, as this material is costly and not easy to find, use a
stainless steel wire which you can buy in a welding shop or in a hardware store. If you
have difficulty finding a wire of this material, use an iron wire. Its diameter should be
of 1 - 2 mm. Even in this case you should use an analytical balance.
Dip the ring just under the surface of the liquid of which you want determine the surface
tension. Level the balance in these conditions. Add some masses on the opposite arm until
the ring detaches from the liquid. The surface tension (T) of the liquid will be given by
the detachment force (F) you have measured divided by two times the mean circumference
(crf) of the ring: T = F/2crf. This factor 2 takes into account the two surfaces of
liquid: the internal one and the external one to the ring (figure 8). For reasons of
clarity, in the figure the ring has been drawn with the diameter greater than the actual
diameter.http://www.tensiometry.com/STMethods.htm
Other methods to measure the surface tension.

7 - With distilled water, verify the good working order of your experimental system.
8 - Determine the surface tension of the tap water.
9 - Determine the surface tension of the tap water to which you have added a little
detergent. You will notice that small amounts of surfactants are sufficient to lower the
surface tension of the water a lot.

10 - Relationship between the weight of the drops and the surface tension.
By a dropper, slowly drop some water of the test 8 and determine the mass of a certain
number of drops (ie 30). Do the same thing with the water of the experiment n° 9. Verify
if there is a relationship between the mass of the drops and the surface tension of the
solutions. Answer: The mass of the drops is proportional to the surface tension of the
liquid: M = T/K, where K is a constant which you can determine using distilled water at
20°C of which you know the surface tension. This constant is valuable only for this
dropper. Determine the mass of a given number of drops is a method to measure the surface
tension of a liquid. In these tests, to obtain a better precision, calculate the mean of a
series of measures. Verify if the following relationships are valuable: T1:M1
= T2:M2.

11 - Surfactant powered boats.
From a thin wooden or cardboard sheet, cut three little "boats" like those
indicated in the figure 9. They must have an opening with a seat for a bit of soap. Place
a bit of soap in the seat of a boat and put it in a small basin with water. You will see
the boat move quickly forward. With the opening on a side or off-center, the boat will
turn. The movement of the boat can be explained by the quick scatter of surfactant
molecules on the water surface, so this little boat would move by reaction. Another
explanation recalls Marangoni's effect, according to which, in case of a gradient of
surface tension from one zone of a liquid to another, there will be established a flow
from the zone of low surface tension toward the one of high surface tension. In this case,
the boat will be dragged by the movement of the water surface. This amusing experiment can
also be done using substances other than soap, provided they have surface active
properties. For example, you could place a little drop of detergent on the carving. If you
will use a bit of camphor, your boat will sail more quickly and longer. If the stretch of
water in which the boat moves is small, like a dish or a small basin, quickly the water
surface will be covered by a layer of surfactant molecules and the boat will stop and you
will need to change the water to restart the boat. If instead you do these experiments in
a pond, you will not have this problem. Try different shapes of boat and of carving, try
hot and cold water, different types of soap, etc. The water will quickly soak through the
wood or especially the cardboard of your boat and will disable it. Some boards will even
sink. To save your fleet, make the little boats waterproof with acrylic paint or flatting.
When the paint dries, you will be able to restart the races.

WETTABILITYWhy does one fabric absorb water well while another seems to refuse it? Why does water
collect into large drops on a greasy surface and instead form an adherent film on a clean
surface? According to the nature of the liquid and the solid, a drop of liquid placed on a
solid surface will adhere to it more or less. To understand this phenomenon it is
necessary to take into account the fact that molecules of a liquid are subject to a cohesive
force which keeps them united to one another, but there is also an adhesive
force which is the force with which the molecules of the liquid adhere to the
surface of materials that they contact. When the forces of adhesion are greater than the
forces of cohesion, the liquid tends to wet the surface, when instead the forces of
adhesion are less by comparison to those of cohesion, the liquid tends to
"refuse" the surface. In this people speak of wettability between liquids and
solids. For example, water wets clean glass, but it does not wet wax.

1 - Measuring the contact angle. Place a drop of a liquid on a smooth
surface of a solid. According to the wettability of the liquid in relationship to this
solid, the drop will make a certain angle of contact with the solid. With reference to the
figure 10, if the contact angle is lower than 90°, the solid is called wettable, if the
contact angle is wider than 90°, the solid is named non-wettable. A contact angle equal
to zero indicates complete wettability. To measure the contact angle use a protractor and
a ruler. Taking a picture of the outline of the drop will make easier and more exact the
measurement.

2 - Prominent drops, flat drops. Lay a water drop on a dirty glass
plate. For example a glass with a lot of fingerprints. Measure the contact angle. Now wash
the plate with water and detergent, then rinse it with care and dry it. Make the test
again and compare the contact angle in the two cases.

3 - Misted plate. Breathe on a glass plate which has been washed, but
not very well. You will see the plate become misted, this is due to the formation of a
myriad of tiny water drops on the surface of the glass.

4 - Water film. With water and detergent, wash a plate of glass well,
then rinse it a first time with tap water and then with distilled water and leave it to
dry in a place devoid of dust. Now, breathe on it. If the plate of glass is very clean, it
will not mist because the water will arrange on the surface as a thin and continuous film
of water. This happens because the water has complete wettability toward a clean glass. If
the cleaning method above has not cleaned the plate well enough, wipe it with a cotton
cloth with some pure acetone in it. Use caution because acetone is inflammable and toxic,
so do this operation outdoors and with care.

By studying plants, a German scientist discovered a method to keep surfaces clean or to
clean them with less water. You have to cover the surface with a thin layer of wax. This
substance has a very low wettability toward the water. It tends to keep clean and it is
commonly used to enhance the cleanliness and appearance of buildings and vehicles.

Let us stay in the field of the wettability. Surely you have noticed that water tends to
rise near the walls of a glass container. This happens because the molecules of this
liquid have a strong tendency to adhere to the glass. Liquids which wet the walls make
concave surfaces (eg: water/glass), those which do not wet them, make convex surfaces (eg:
mercury/glass). Inside tubes with internal diameter smaller than 2 mm, called capillary
tubes, a wettable liquid forms a concave meniscus in its upper surface and tends to go up
along the tube (figure 11). On the contrary, a non-wettable liquid forms a convex meniscus
and its level tends to go down. The amount of liquid attracted by the capillary rises
until the forces which attract it balance the weight of the fluid column. The rising or
the lowering of the level of the liquids into thin tubes is named capillarity.
Also the capillarity is driven by the forces of cohesion and adhesion we have already
mentioned.

1 - The rise of water along a capillary. Immerse a capillary in a
glass containing tap water and measure the height (h) of the water column inside it.

2 - Effect of the surfactants. Add a few drops of detergent to the
water and measure again. Compare the variation in the height of the water column. You will
be able to notice that even small amounts of surfactants produce important effects on the
level reached by the water in the capillary.

3 - Effect of the diameter of the capillary. With a tube of glass and
a Bunsen burner, make a series of capillary tubes having different diameter. Verify the
relationship between the height of the water column and the internal diameter of the
capillary. (Answer: the height of the column is described by this formula h=k/r, where h
is the height of the column, k is a constant which depends on the surface tension of the
liquid and on the contact angle between the liquid and the wall, r is the internal radius
of the capillary tube. So, with the same liquid and material of the capillary tube, the
height of the column is in inverse proportion to the diameter of the capillary tube. You
can determine the value of k for water using distilled water at 20°C.

4 - Try other liquids. Make some other tests with liquids other than
water, such as alcohol, oil, etc. and measure the height of the liquid column. This height
depends by a number of factors such as the surface tension of the liquid, the contact
angle liquid/capillary, the radius of the capillary, the density of the liquid, the
acceleration of gravity. In fact, the column attains the height of equilibrium between the
ascensional forces and its own weight. Oily substances tend to contaminate inside the
capillary, so when changing from one liquid to another, clean the capillary well or
replace it. The vegetable world exploits capillarity and osmosis to bring water up to the
higher parts of plants. In this way, some trees succeed in bringing this precious liquid
up to 120 meters above the ground.

5 - An emergency plant watering system. It is summer and you are going
on vacation. You are worried about your potted plants, which risk to remain without water.
In fact, even if you have asked your neighbor to water them, you know by experience that
after the first day, he will forget, that's just the way he is. Then, try this emergency
watering system. It bases itself on the fact that a string is able to carry water among
its fibers by capillarity. Place a tank on some bricks and fill it with water. Place the
pots round the drum. Cut some pieces of string long enough to reach the bottom of the tank
and to be inserted into a pot. Immerse all strings in the water to soak them well. Tie all
the strings together at one end and sink this knot to the bottom of the drum with a stone
or weight. Now, one at the time, put the free end of each string into a different pot.
Each pot has to be served by a string. Test the system before you go on your vacation. You
have to verify if it works well, to find the suitable type of string and to proportion the
amount of water in the tank to the length of your absence. Try strings made up of fibers
of different dimension, of different materials, even in plastic. If the string tends to
become encrusted with mineral deposits, add some vinegar to the water. Also try to insert
each string in a thin plastic tube. If the water flow is too fast, use a thinner string.
Check the effect of some drops of detergent on the flow.

SOAPS AND DETERGENTSHow do soaps and detergents work in removing dirt? Soaps and detergents are formed by
special molecules, which have a hydrophilic head, which therefore loves to remain in water
and a hydrophobic tail, which avoids water and loves fat substances (figure 12 A). Because
of their hydrophobic tail, a part of the molecules of detergent collects to the water
surface forming a monomolecular layer (figure 12 B), it lowers the surface tension of the
water and makes easier its penetration into the fabrics to be cleaned. Within the water,
the molecules of detergent collect themselves in micelles and membranes,
little aggregates of molecules united by their hydrophobic tail (figure 12 B). When they
meet dirt, these molecules surround the particles and insert their tail in them. The
hydrophilic heads attract the dirt toward water and with the agitation of the liquid they
contribute to remove the dirt from the fabric (figure 12 D). The crown of hydrophilic
heads carries the particles of dirt in the water (figure 12 D), where they end up in
suspension and then they are rinsed away. Hence, the dirt water contains also greasy
particles which have been emulsified. For the same reason, the detergents aid the
formation of emulsions. The substances which lower the surface tension of a liquid are
called surfactants (from: surface-active agents). The lowering of the
surface tension of the water allows the formation of soapy membranes (figure 12 C), foam
and soap bubbles. Notice the special arrangement of the surfactant molecules in these
membranes.

The phospholipids are molecules like surfactants, they also have a hydrophilic
head and this time two hydrophobic tails. These molecules are the main components of the
membranes of cells. In fact, usually the membranes of cells are made up of two layers of
phospholipids, with the tails turned inward, in the attempt to avoid water. As we know,
the external membrane of a cell contains all the organelles and the cytoplasm. Liposomes
are empty cells which are manufactured by some industries. They are microscopic vesicles
or containers, formed by the membrane alone. They are widely used in the pharmaceutical
and cosmetic fields because it is possible to insert chemicals inside them. You can use
liposomes to contain hydrophobic chemicals such as greasy or oily substances so that they
can be dispersed in an aqueous medium by virtue of the hydrophilic properties of the
membrane of the liposomes.http://cellbio.utmb.edu/cellbio/membrane_intro.htm
Membrane Structure and Functionhttp://ntri.tamuk.edu/cell/membranes.html
Architecture of membranes
Internet keywords: phospholipids membrane, cell membrane

1 - Comparison of the ability of different detergents. Try the
efficacy of different detergents for glass or dishes. Soil some microscope slides with the
same type of fat. If you do not have microscope slides, use glasses or even ceramic
dishes. Clean all the slides with a different detergent, rinse them well and dry them. You
can check the level of cleanliness by measuring the contact angle of water drops placed on
them. Another method is to measure the reflected light by each slide in the same
conditions of illumination by means of an exposure meter: the cleaner slide reflects less
light.

SOAP BUBBLESAs long as there has been soap, making soap bubbles has been an amusement for children.
Everybody has played with soap bubbles as a child. A straw and a glass with soapy water is
all that is needed to amuse a child for hours. One child blows bubbles and others run
after them and play with or pop them. What astonishes the children is the spherical and
perfect shape of the bubbles, their colors, their transparency, their lightness which
competes only with that of the butterflies and fairies. By means of thin membranes of
soapy water, it is possible to do interesting experiments and amusing games, such as to
blow bubbles of different sizes, concentric bubbles, helical bubbles, "solids"
supported by frames in metal wire, it is possible to observe and to study the coloured
interference figures on the membranes of soapy water, to obtain membranes so thin that
they lose all color and become invisible, to obtain membranes measuring some square meters
of surface and bubbles of some cube meters of volume, so that you can to trap a friend.
And then you will learn to blow cubic bubbles... by using a square straw, of course! No,
just kidding! :)

HOW DO THE SOAP BUBBLES FORM?
As Grownups, we pose questions like these: "How do soap bubbles form? Why does soapy
water produce foam while pure water does not?". When water sprays from a tap in a
small basin, you can see bubbles form, but they burst very soon. This is due to the fact
that the surface tension of the normal water is high and it tends to draw the water
molecules into the main body of the water, to the point where the thickness of the bubble
wall is too thin to remain intact and quickly bursts. Instead, the surface tension of the
soapy water is much lower: about a third of the pure water, so the molecules of the bubble
are less stressed and it can last longer. Soap and detergents lower the surface tension of
water and, as we have said, they are called surfactants. As we have said in the paragraph
on the soaps and detergents, the molecules of surfactants have a hydrophilic head and a
hydrophobic tail. When these molecules are dissolved in water, they tend to collect on the
surface with the tails outward, forming continuous layers (figure 12 B). The membranes of
soapy water are made up by three layers: the external two are formed by surfactant
molecules and the internal layer is formed by soapy water (figure 12 C). These layers of
surfactant molecules are very elastic and they deform easily without breaking. They also
slow the evaporation of the water film and so extend the life of the bubbles.

RECIPES
Water is an important ingredient to our recipes. Usually, to produce soap bubbles, people
used a mixture of tap water and soap. Unfortunately, the mineral salts which make hard
water subtract a part of soap with negative consequences on the formation of the bubbles.
In fact, soap reacts with the calcium and magnesium salts, which are in the tap water,
forming an insoluble precipitate which subtracts surfactant molecules from the solution.
Instead, the detergents react with the mineral salts of the water producing soluble
compounds, so detergent are less influenced by the hardness of water. If your tap water is
soft, it is OK to use for bubbles. In any case, you will obtain the best results with
distilled water.

After the water, the most important ingredient is the base surfactant. There are a lot
of surfactants which can be used as detergents and to blow bubbles. Therefore, try some
different brands of detergent until you find the best one. Dawn and Joy brand liquid
detergents for dishes supplied good results, but try other products if you like.

The presence of water in a soapy film is important to make it last a long time. As time
goes by, a part of the water migrates by gravity and reaches the bottom of the film or of
the bubble and another part evaporates. In this way, the membrane grows thin, weakens and
in the end bursts. To extend the life of bubbles, people add substances which make the
water more viscous, slowing its descent toward the bottom. Other substances are added to
slow the evaporation of the water. Substances which have these effects are: sugar, honey,
glycerin, gelatin, arabic gum, viscous liquid soap. You will have best results if you let
the soapy solution rest for a couple of days, but if you are impatient, you can use it
immediately. A cold solution makes longer lasting bubbles. For various bubble recipes,
look at the links we have put at the end of this section on bubbles.

1 - How to find the basic surfactant.

To
find the main component of your recipe, the base surfactant, obtain some dishwashing
detergents, shampoo, bath soap, etc. With water, make a solution in the ratio of 1 to 10
for each surfactant. In a place without wind, blow a bubble of about 7 cm in diameter.
Keep it on the straw (figure 13) and measure its duration. Repeat the test 5 times for
each detergent so to obtain a more reliable mean value. Obviously, the best detergent is
the one which produces bubbles which last longer.

Figure 13 - How to keep the bubbles during the test of duration.

2 - Adjusting the secondary ingredients. A second series of tests will
have the purpose of adjusting the recipe in its secondary components, those destined to
reduce the evaporation and the fluidity of the water. Follow the same method as you did in
point 1.

3 - Blow some bubbles. When the solution is ready, you will be allowed
to pass to the further experiments. In the meantime, blow some bubbles and watch them fly,
carried by the wind.

4 - How to make bigger bubbles. With some thick iron wire, make a ring
of about thirty cm diameter. Immerse it in bubble solution that you have put in a small
basin. Moving the ring quickly in the air, you should be able to obtain quite large
bubbles.

5 - Again on the force of the surface tension.

Knot a heavy cotton thread with a slipknot to the ring of the experiment 4. After you have
wet the ring in the soapy solution, the ring will be closed by a film. If you burst the
membrane inside the loop, you will see it take a circular shape (figure 14). This happens
because of the surface tension of the remaining part of the soapy film.

6 - A
support for bubbles. To comfortably observe bubbles, it is important they are
steady. With some iron wire, make some rings on which to put the bubbles. Leave a stem to
each ring so you can insert it into an object or you can shape as a pedestal. To avoid
bursting the bubbles you put on it, wet the ring with bubble solution. Wood or velvet can
support bubbles for a long time without bursting them, but are harder to fashion into a
ring shape.

7 - Study the contact surface among bubbles. On a clean glass or a
rigid plastic sheet soaked with solution, place two bubbles in contact each other. Observe
the surface of contact. You will see the smaller bubble of the two will tend to bulge into
the bigger one. This happens because of the internal pressure of the little bubble is
higher than the pressure of the large ones. This also means that two bubbles of equal
diameter have a flat contact surface. After having made some bubbles in contact with each
other, produce some foam and observe it. Observe that sometimes the shapes of the foam
bubbles are the same as that of cells of biological tissues, in other cases the shapes of
the cells are different because they have to increase their surface of contact or for
other reasons. Note also that the crystals of metals often have the same shape as the foam
bubbles. After all, during the solidification of a metal, they are deformable spheres very
close each other and which cannot leave empty spaces.

Figure 15 - Membranes on a cubic frame. These
membranes do not arrange on the faces of the cube, but they are in contact each other.

Figure 16 - Membranes on a cubic frame. The cubic
central bubble has been placed with a straw.

Figure 17 - Membranes in a pyramidal frame
(tetrahedron). Place a bubble in the center.

Figure 18 - Membranes between two rings and
having a film in common.

Figure 19 - Tube-shaped membrane between two
rings. It has been obtained by breaking the film in common.

8 - Solid figures made on suitable frames. With some frames made with
metal wire, you can create flat, helical films or with many other forms. You can also
create quite complex solids (figures 15, 16, 17, 18, 19, 20). To do this you have to dip a
suitable frame into the soapy solution. When you will have withdrawn it, you will see the
membranes. Usually, people expect these films to form on the faces of the solid, but this
does not happen because they tend to keep into contact with each other and to form figures
of minimum surface area. Remember that soapy films tend to keep the shape of smallest
energy. So, if you will make a tube-shaped membrane, do not be surprised if its
diameter will reduce in the middle.

9 - Helical films. To obtain helical films (figure 20), make a helix
with a few coils made up of iron wire (like a normal spring), place a piece of wire along
the axis of the helix and solder it to the two extremities of the helix.

You can bet your friends you are able to make cubic soap bubbles. Obviously, they will
not believe you. Then you can explain them that this is possible to you by using a square
straw. It is very probable they will accept the bet. It will be easy for you to win it by
making your cubic bubble inside a cubic frame, as shown in figure 16. Before blowing the
bubble, crush the tip of a straw so to obtain a square section. This is part of the bet,
but you know that with a normal round straw that the bubble will become cubic due to the
frame, not the straw.

Figure 20 - Helical film.

Figure 21 - Frames on metal wire to study the soapy
membranes.

The figure 21 shows some frames of metal wire which can be made to study the soap films
and to measure the surface tension of liquids. To build them, we have used galvanized iron
wire, cut in segments which then we have soldered with tin. You can also try plastic
coating these frames by dipping them into tool handle coating products which are sold at
hardware stores.

"Why are soap bubbles colored?". The membrane of the soap
bubbles are formed by three layers. The external two are both formed by a layer of
surfactant molecules with the polar head turned inward, the inner layer is formed by soapy
water (figure 12 C). The light which crosses a soap film is in part reflected by the front
surface of the membrane and by the back one. The waves of light reflected emerge out of
phase, they sum algebraically (interference), giving rise to variations of color. The
emerging hue depends on the thickness of the film. These colors are very fine and create
beautiful shapes formed by the zones of different color when turbulence is present within
the film. In fact, if you gently blow on a film, you can create magnificent designs
(figures 1, 22, 23, 24). Over time, due to evaporation or the descent of the water toward
the bottom, the thickness of the membrane will have become very thin, the two reflections
will fade completely and the bubble will become black against a black background: it will
not show colors any more and will become invisible. In that condition, the film will be
also very unstable and near bursting.

Figure 22 - The interference fringes which form as the water flows down by
gravity. As the film gets thin at the top it becomes black because its thickness is less
than the wavelength of the visible light.

Figure 23 - By gently blowing on the film, you can create
beautiful turbulence zones which can be observed and studied. Notice on the top the black
zone has widened.

Figure 24 - By blowing again, the figures become more complex
and rich with details.

11 - Colors and shapes of the figures of interference on soap films.
The soap membranes are well suited to observe the colors and the turbulences which are
created by light air currents. So, by means of a ring on iron wire, make a soap film and
examine its colors. Blow lightly on the film to observe the turbulence on its surface
(figures 23 and 24). To better see the colors of the membrane, it is worthwhile to observe
it against a black background and illuminate it with bright white light. If you keep the
frame vertical, you will see the colors change as the film grows thinner. Usually, shortly
before bursting, a part of the film will become black. Here some other figures of
interference: figure 31, figure 32.

12 - To cross a membrane without bursting it. If you touch a film with
a dry finger, the membrane will burst. If you will wet the same finger with the soapy
solution, the film will not burst and you will be able to penetrate it.

13 - Plays on the water. Make bubbles in a small basin of water. Look
for the conditions which allow to the bubbles to bounce or to alight on the surface
without adhere to it. Place a drop of oil on the surface of the water, which will arrange
itself on the surface as a monomolecular layer, (eg. stearic acid) and repeat the test.
Also an oily hair can deposit a thin, oily layer on the waters surface, when slowly
immersed in it.

OSMOSISIf you place two solutions of different concentration side by side, keeping them separated
only by means of a membrane, you will see the level of the more concentrated solution
increase (figure 25). This happens because the two solutions try to attain the same
concentration by diffusion. The membrane has to be semipermeable, that is it has to allow
the passage of the solvent but not of the solute. The molecules of the solvent have to be
smaller than those of the dissolved substance. In practice, this condition is very
frequent given that the molecules of water are very small. It is necessary to remember
that it is possible to make solutions with other liquids also. Osmosis is the tendency of
the system to reach the same concentration in both solutions. It is a phenomenon of great
importance in biology and which is also the basis of the function of the kidney, of the
absorption of water by plants and which is used by industries to concentrate or to purify
solutions. In fact, applying a pressure on the side of the more concentrated solution, it
is possible to reverse the process and cause the solvent to pass to the less concentrated
solution. This is the process of the reverse osmosis. It is used also to purify
water, to concentrate solutions, etc.

In order to do experiments with osmosis, you need to obtain a semipermeable membrane.
For this purpose, you can use cellophane,
which is a thin transparent film, essentially made up of cellulose and which is often used
to pack wrap flowers and gifts. Sometimes, florists also use a plastic which is very like
cellophane, but, instead is completely impermeable to the water and which is not suitable
for these experiments. How can you distinguish between these two materials? Putting some
water on cellophane, you will see it soften, dilate and even the opposite side of the
sheet will become moist. This does not happen with the transparent plastic sheet. You can
obtain cellophane in a stationery shop. Unfortunately, this material is often covered with
a thin layer of water repellent nitrocellulose which prevent the passage of the water.
This layer can be removed by immersing the cellophane in a solvent for varnish or perhaps
in acetone. Use caution because these solvents are inflammable and toxic.

Another possible source of semipermeable membrane can also be found in certain plastic
bags. The plastic is made from starch and is used to produce biodegradable plastic bags
for recycling. In some European cities, these plastic bags are used to collect organic
wastes. When touched, this plastic is flabby, quite elastic and near rubbery. You can also
try the membrane of a chicken egg and other membranes you will find or you are able to
fabricate.

Water flows slowly through the membrane. If you limit yourself to closing the bottom of
a tube, it will take days to see the level of the inner liquid increase. To accelerate the
flow, it is necessary to widen the surface of exchange. It would be necessary to have
special flared tubes, which are difficult to find. Instead, you can use a small funnel,
which is much easier to obtain.

1 - Diffusion by osmosis. For the first experiment, use distilled
water, some sugar, a semipermeable membrane, a beaker, and a support for pipettes. Obtain
a flared tube of glass or transparent plastic. Or, as an alternative, a little transparent
funnel. The internal diameter of this tube has to be at least one cm. With a rubber band
or clamp, attach a piece of membrane on the flared bottom of the tube and then pour the
concentrated solution of sugar in the tube. Insert the tube in a beaker and put water into
it until you attain the same level of the solution in the tube. After some hours, you
should see the level of the liquid in the tube is increased (figure 25). After some time,
the level will attain a maximum. If, instead of tap water, you will use distilled water,
the phenomenon will be more evident. To render more visible the concentrated solution, you
can add a drop of ink or some watercolor. Why does the more concentrated solution rise? As
we said, there is a tendency of the two solutions in contact via a semipermeable membrane
to reach the same concentration. The more concentrated solution absorbs solvent from the
more diluted. In these experiments, the level of the liquid in the tube increases, but not
to infinity. It goes up until the pressure of the liquid column attains the equilibrium
with the osmotic pressure. The equilibrium pressure between a solution and its solvent is
the osmotic pressure of that solution.

2 - Osmotic pressure and density of the solution. Determine the
osmotic pressure of some solutions. Verify if it is proportional to the amount of
molecules per volume of the solution.

3 - When the dissolved particles are very small. If, instead of the
sugar, you will use salt, the osmotic pressure will result very low. This happens because
in water the salt dissociates itself into the Na+ and Cl- ions, which are smaller than the
molecules of water and they easily pass through the semipermeable membrane.

4 - Osmotic pressure and microorganisms. Place under the microscope a
slide with a small drop of water rich in protists, then add a pair of drops of distilled
water. At the beginning, the protists will swell and you will see their vacuoles work very
hard in the attempt to expel the excess water from their cytoplasm, then you will see
their cellule explode, pouring their organelles outward. The cilia of the mouth will
continue to beat for long time, even if they are not connected to the body any more.

INTRODUCTION TO THE COLLOIDAL SYSTEMSLet us leave the surface phenomena to enter into the mysterious world of the colloids. A
first example of a colloid is gelatin, a strange substance: neither liquid nor solid. It
is very elastic and if deformed it returns to its previous shape. Goofy, the friend
of Mickey and Donald, learned something about it when, in the Disney film: Mickey and
the Beanstalk, he was "walking" on a pudding of the Giant. The emulsion of
oil in water is another substance with unusual properties. Unusual are also substances
such as foams, aerosols, smokes and fogs, not to mention the solid emulsions and foams.
What do all these curious substances have in common? That is what we will see before long.
These substances are called colloids and they are in some ways related to
the solutions and to the mixtures, even if they do not
belong to the former nor latter. To understand what colloids are, it is necessary to know
what solutions and mixtures are.

SOLUTIONSA solution is a homogeneous mixture of two or more substances. When placed in water, many
substances dissolve and are called soluble, others do not dissolve and are called
insoluble. Salt and sugar easily dissolve in water. If instead you put sand in water, you
can mix for as long as you want, but you will not succeed in dissolving the sand. In fact,
sand is insoluble in water. In a solution, the material present in greater quantity is
defined solvent and that in smaller quantity solute. What does it mean to say that a
substance is soluble in another? It means that the molecules of the solute separate each
other and they disperse among those of the solvent. Instead, the insoluble substances keep
themselves compact and their molecules do not disperse into the solvent. As solvent, we
have used the example of water because many solids are soluble in water, but nearly every
liquid can be a solvent. And then, why we should limit ourselves to the liquids? Let us
generalize the concept of solvent and concede to all substances, solid or liquid or
gaseous the possibility to be a solvent. At this point, even the solutes can belong to all
of these three states of matter. For example, some solid solutions are the metal alloys
such as steel (Fe+C), brass (Cu+Zn), bronze (Cu+Sn). Finally, all gases are completely
soluble among each other. Also common are solutions of gases in liquids. For example,
carbon dioxide is added to many beverages to make them fizz. In the water of ponds, rivers
and seas, gases like oxygen, carbon dioxide and others go into solution in a natural way.
The presence of these gases in the water make possible the life of the aquatic organisms.

The solubility of a substance is measured as the maximum amount, in grams,
which can be dissolved in 100 g of solvent. When the solute does not dissolve any more,
but a deposit is formed on the bottom, the solution is defined saturated.

CATEGORIES OF SOLUTIONS

SOLUTE

SOLVENT

EXAMPLE

Gas

Gas

air (nitrogen, oxygen, etc.)

Liquid

Gas

moist air (water vapor in air)

Solid

Gas

atmospheric dust

Gas

Liquid

CO2 in water (sparkling water)

Liquid

Liquid

wine (water + alcohol)

Solid

Liquid

marine water (salt in water)

Gas

Solid

gas in silicates (pumice stone)

Liquid

Solid

dental alloys (mercury in cadmium)

Solid

Solid

metal alloys (steel, bronze)

1 - Saturated solution. Determine the content of salt in a saturated
solution. In order to not waste too much salt, use only a little water.

2 - To grow crystals. Determining the density of sugar in a saturated
solution is not easy because sugar continues always to dissolve. Anyway, make a heavy
sugar solution and a saturated solution of salt in water. Put a cotton thread in each of
them and wait some days for some crystals to grow. Describe the shape of these crystals.
If you like to grow crystals, it is possible to find packets of salts specially chosen to
this purpose. Also search the Internet with the words: growing crystals.

3 - Where does sugar go? Put a beaker on a magnetic stirrer, insert
the stir bar and fill the container with water up to the top. Slowly, add grains of sugar
so they are dissolved by the stir bar as it rotates. Note the amount of sugar you will
have put into the water before it overflows. Do the same thing with salt and then with
sand. Compare the results and explain the different behaviors.

4 - How to separate salt from sand? Solve this problem: A day, a child
who lived on the border of the desert was sent to buy some salt. While he was coming back
and he was playing with friends of his own, the bag broke and the sand shed on the sand.
For these people the sand was important and costly, so that child would be scold by his
parents. How would have you done to recover the precious salt, separating it from the
sand?

MIXTURESAs we have seen, by mixing sugar with water, a solution is obtained. If instead we mix
sand into water, we obtain a mixture. Also by mixing bits of coal and iron filings we
obtain a mixture. With a pair of thin tweezers it is possible to take away sand grains
from the water or pieces of coal from the filings, but it is not possible to take away
singly molecules of sugar from the water because they are too much small. Hence, what
distinguishes a mixture from a solution? In a mixture the particles are enough large to be
separated by mechanical means such as tweezers or sieves, in a solution this is not
possible because the particles which form it are so small that they cannot be seen even
with an electron microscope. To separate the components of a solution it is necessary to
use physical method like distillation. So, mixtures are formed by quite big particles,
solution are formed by very small particles.

1 - A mixture. Make a mixture, for example by using sand and wood
sawdust. How could you quickly separate the two components?

2 - Sedimentation speed and size of the particles. As indicated in the
experiment on the analysis of the soil
composition in the article on the experiments on environmental education and biology,
put some water and a sample of earth in a glass or transparent plastic jar. Close the pot
and shake it until all the earth is dissolved. Place the jar at rest and observe the
different layers of materials. On the bottom, there will be stones and gravel, then thick
sand and fine sand. Silt will require half an hour to be deposited, clay will demand 24
hours. Very small particles will remain in suspension, some of them will deposit very
slowly, the finest ones instead will never deposit. Some other substances will have gone
into solution. It seems the Etruscans collected the very fine clay which deposited after
some days to obtain the black color of their earthenware.

3 - To separate particles according their grain size. If you want to
separate the thick sand from the finer sand, you can use a sieve. If you want to clean
sand from silt and clay, you can use flowing water. With a plastic tube, make water flow
into the container of the sand. The water will carry away the smaller particles, while the
larger ones will remain in the container. This method exploits the different sedimentation
speeds to separate the particles of different grain size. Usually, the sand destined to be
put in aquariums is cleaned to avoid water contamination. By using a sieve and with
sedimentations and cleanings, produce 100 g of thick sand, 100 g of thin sand, 100 g of
silt and 100 g of clay. Remove the water in excess and let all components dry to obtain
moist sands, soft silt and clay. Compare the properties of these materials.

4 ­ Observe under the microscope the finest particles. With a
microscope, try to measure the size of the particles of silt, clay and of those which
remain in suspension in water during your experiments of sedimentation.

COLLOIDSWe have seen that in the solutions, the molecules of the solute separate each other and
disperse among those of the solvent. In the mixtures instead, the molecules do not
separate and the particles remain compact. From the point of view of the sizes, solutions
are formed by very small particles (single molecules) and the mixtures by quite large
particles. In an intermediate position, between mixtures and solutions, there are the
colloids. They are dispersions of small particles, but not molecule sized. What
distinguishes mixtures from colloids and from solutions is therefore the size of
the particles which form them. By convention, a colloid is a dispersion of
particles which size is comprised between 0.2 and 0.002 µm (a micrometer, or micron, = 10-6
meters). If the particles are larger than 0.2 µm, we have a mixture, if they are smaller
than 0.002 µm, we have a solution. In general, the components of a colloid are formed by
small aggregates of molecules, while the components of a solution are single molecules.
Anyway, if these molecules are large enough, as it is the case of many macromolecules,
their solution will give a colloid. So, the criterion of distinction between colloids and
solutions cannot be the presence of single molecules, but as we were saying, the size of
the particles which form them.

MIXTURES

COLLOIDS

SOLUTIONS

large particles

> 0.2 µm

mean particles

0.2 - 0.002 µm

thin particles

< 0.002 µm

According to the dispersing phase, colloids are distingued in gaseous, liquid and solid
suspensions. Gaseous suspensions, or aerosol, are smokes and fogs. Smokes are suspensions
of solid particles in a gas. Fogs are suspensions of liquid particles in a gas. Sols,
gels, emulsions, foams are liquid suspensions. Oily rocks, pumice stones are solid
suspensions.

TYPES OF COLLOIDS

DISPERSED PHASE

DISPERSANT PHASE

NAME

EXAMPLE

Solid

Gas

Smoke - Aerosol

Smoke

Liquid

Gas

Fog - Aerosol

Fog

Solid

Liquid

Sol, Gel

Paint, Gelatin

Liquid

Liquid

Emulsion

Milk

Gas

Liquid

Foam

Beer foam

Solid

Solid

Solid suspension

Amethyst

Liquid

Solid

Solid emulsion

Oily rocks

Gas

Solid

Solid foam

Pumice stone

The term colloid refers to substances with a glue-like consistency, in which the
dispersant phase is therefore liquid. However, do not forget that even substances such as
smokes and aerosols, in which the dispersant phase is aeriform and which we can also call gaseous
suspensions, are colloids. Finally, even some solid substances, in which the
dispersant phase is solid and which we can also call solid suspensions, are
colloids too.

Colloids have unusual properties, for example gelatin. Colloidal systems have a high
ratio area/volume among the surface of the particles and their volume. In other words, as
in the colloids the amount of dispersed particles is very large, their overall surface is
very large too and by consequence the interaction of the two phases is important. For
example, a cube of 1 cm a side has a surface area of 6 cm2, the material of the
same cube divided into little cubes of 0.002 µm of side, has a surface area of 3000 m2.
Because of the wide surface of contact between the two phases, often the colloids are
studied with the surface phenomena and the discipline which studies them is called surface
and colloid science.

SOLA sol is a dispersion of very thin solid particles in a liquid. It has a liquid
consistency and resembles a true solution. An aqueous sol appears clear, very similar to
common water. Anyway, if you shine an intense beam of light across it, a part of the light
will be diffused from the particles which are in suspension. These particles are very
small, but they are still enough large to obstruct the light and diffuse it. This
phenomenon is called Tyndall effect. You can observe it with sols, but not with true
solutions.

1 - Tyndall effect. In a transparent jar, put some clayey earth 1/4 of
the volume and water until attain 3/4 of the container. Close the jar with its cap and
shake until all the earth is "dissolved". Leave the pot to rest for a day to
allow the clay particles to settle. The liquid which is above the sediment should have
become clear. Shining an intense bundle of light through the jar, you should see the
Tyndall effect. Do the same thing with a glass of pure water and compare the results.

GELA gel is a dispersion of very thin solid particles in a liquid and it has a gelatinous
consistency. Increasing the concentration of the particles, a sol can pass to the state of
gel. On the contrary, by diluting a gel you will obtain a sol. So, what makes a sol
different from a gel is its fluid or gelatinous consistency. Also the temperature can
determine the passage from sol to gel and vice versa. For example, broth gelatin is
gelatinous at room temperature, but it becomes liquid when it is heated. Animal gelatin is
a reversible gel because depending on the temperature it can pass from gel to sol and vice
versa The albumen of eggs instead is not reversible because when heated it coagulates and
it does not come back to the state of sol. Silica gel absorbs moisture and keeps its
properties with broad concentrations of water. Because its affinity for water it is used
as dehumidifier. When left to rest, a sol can spontaneously jell and come back to the
state of sol simply by mixing it (eg: aqueous suspensions of kaolin).

1 - Making gelatin. Buy some dry gelatin. Dissolve it in warm water and,
with subsequent dilutions, determine what is the minimum concentration of dry
gelatin necessary to obtain a normal gelatin at room temperature. Do not keep
gelatins a long time because they easily become cultures of bacteria. Store them in a
refrigerator and, after a day, throw them away.

2 - Reversibility of the gelatin. By means of the temperature, make
some gelatin pass from the gel to sol states and vice versa.

3 - Experiments with vegetable resin. Resins are gels and they possess
useful properties. Often, fruit-bearing plants produce gelatinous spheroids which diameter
can attain some centimeters. Conifers are important producers of resins and often you can
collect drops of resin which hang from their trunk. You can also make an incision on a
trunk to obtain some resin. Canada Balsam is a very important resin in optics and in
microscopy. It is extracted from the Abies balsamea, a conifer of North America
and it is used to glue lenses and to make permanent microscope slides. For their adhesive
properties, resins take part to the composition of paints. Collect resin from trees,
observe under the microscope the particles which are suspended in it. Dissolve the resin
of a fruit-bearing tree in warm water and try to obtain a glue. Dissolve the resin of a
conifer in turpentine and assess their adhesive properties.

4 - Experiments with polysaccharides. Polysaccharides are resinous
gums soluble in water. They are used in the fabrication of cosmetics, paper and in a lot
of other applications. Some polysaccharides are edible and are added in creams, yogurts
and in other foods. You can obtain some polysaccharides and experiment with their
properties. In particular, add to them some water and check the consistency, viscosity and
adhesiveness of the substance you will obtain.Absolutely do not eat polysaccharides, do not inhale their
powders and do not use them in recipes for food. If eaten dry, these
substances will swell and risk obstruction of the digestive tract. If inhaled, they will
swell and risk obstruction of the respiratory airways, causing dangerous problems in
breathing. Do not use them in food recipes, but only in experiments. Keep in mind that
some polysaccharides are not edible. When hydrated, these substances become culture medium
for bacteria, so use them for a short time and then throw them away. An adult must be
always present during these tests.http://saps1.plantsci.cam.ac.uk/worksheets/ssheet22.htm
Some Gum Fun (experiments with polysaccharides).http://food.orst.edu/gums/foegeding.html
Hydrocolloids, Vegetable Gums References. http://class.fst.ohio-state.edu/FST605/lectures/lect20.html
Gums and stabilizers (formula and other information).
Internet keywords: polysaccharides, hydrocolloids, experiments, recipes.

5 - Making photographic gelatin. Photographic gelatins have a
suspension of silver halide salts, which are sensitive to the light. When they are still
warm, these gelatins are spread on a transparent plastic film to obtain a photographic
film, or on a card to obtain paper for photographic prints. As shown through the history
of photography, there are many methods to produce photosensitive surfaces, and many of
them do not use silver salts. In the Internet you can find recipes to make photosensitive
films and paper by many techniques. These preparations require the use of substances and
procedures which can be dangerous. Read information on the caution needed. Children must
be guided by an adult who is expert in chemistry.http://www.cheresources.com/photochem.shtml
Chemistry of Photographyhttp://www.astro.wisc.edu/~mukluk/misc.html
Miscellaneous Photographic Formulas and Information.http://www.tri-esssciences.com/photography_books.htm
Photography books of recipes.
William Crawford; The Keepers of Light : A History and Working Guide to Early Photographic
Processes (a book).
Internet keywords: photographic gelatin recipe / formula, photography sensitizing
processes, photography chemistry.

EMULSIONSAn emulsion is a dispersion of an insoluble liquid in another liquid. For instance, the
oil is not soluble in water. If you pour some oil in a container with water, it will float
it and keeps separate from the water. Instead, if you vigorously shake the container, you
will obtain a dispersion of small drops of oil in water, however these drops quickly join
together, so that in a short time nearly all the oil will return as before. To make the
emulsion more stable, before shaking the container, add some detergent. The surfactant
molecules will arrange on the surface of the oil drops with the heads outward. As these
heads have an electrical charge and as this charge is always the same, the oil drops will
repel each other and be unable to return to the homogeneous layer as before. So,
surfactants can help you to obtain more stable emulsions. There are special surfactants
for emulsions, endowed of a higher capability to stabilize the oil drops than the
detergents. There are also emulsifying agents for alimentary use such as lecithin and
emulsifiers for industrial purposes which are not edible. Butter is formed by small water
drops suspended in fat. Cheese and mayonnaise too are considered emulsions. A lot of
creams used both in pharmacy and in cosmetics are emulsions. Fuels emulsified with water
have been produced. Emulsified oils are used in machine working to make it easier to cut
metals with machine tools. In fact, metal cutting can create an intense heat, which has to
be removed if you want to avoid burning the tools. The oil and water in the cutting fluid
help remove the heat and make it possible to cut metals efficiently. Milk is another
emulsion made up by small greasy drops in an aqueous phase.

1 - Stability of the emulsions.

Fill two
plastic bottles halfway with water, then put 5 cc (about a spoonful) of vegetable oil in
each. Only in one of these bottles, put 0.5 cc (about 20 drops) of liquid detergent for
dishes. Close the bottles and shake them for a couple of minutes to emulsify the oil, then
place them on a table and observe them. The drops of oil will try to reassemble and to
surface. By comparing the two emulsions, you will see that the one with detergent will be
much more stable (figure 28). In fact, even after a month, the white color of this
emulsion indicates that there is a great deal of small oil drops in the liquid, while in
the other bottle the liquid is become nearly transparent, this is a sign that near all the
oil drops have fused together and surfaced.

2 - Vinegar and vegetable oil.
Using a kitchen whisk, emulsify a teaspoon of vinegar with 125 cc of peanut oil or olive
oil. The emulsion will result instable.

Figure 28 - The two emulsions of the
experiment 1 after 24 hours of rest. In the right bottle, some detergent has
produced a more stable emulsion.

3 - Mayonnaise. To the ingredients of the test 2, add an egg yolk and
emulsify again. The emulsion will be much more stable. Add some salt and if you want some
pepper and you will have obtained a good mayonnaise. If you prefer, you can replace the
vinegar with lemon juice. Why is the emulsion stable with the egg yolk? This is due to the
presence of lecithin in the egg yolk. Lecithin is a surfactant and the molecules spread on
the surface of the oil drops with the hydrophilic head outward. As these heads are
electrically charged, the oil drops will repel and their merging is prevented.
Lecithin is a phospholipid and it has a structure like that of the phospholipids which
form the membranes of cells. Another well known lecithin and which you can find on the
market is soy lecithin.

Foam is a dispersion of a gas in a liquid (liquid foams) or in a solid (solid foams).
Among the liquid foams, we have the ones produced by soaps and detergents, and various
foods such as wine, beer and many others. Among the solid foams we have Pumice stone,
earthenware, sponges, expanded plastics like expanded polystyrene and expanded
polyurethane. By dispersing helium in a liquid which produced bubbles with very thin walls
and which then solidified, some researchers succeeded in fabricating a solid foam lighter
than air.

1 - Foam and shape of the bubbles in contact. With a drop of liquid
detergent in a small basin of water, make a foam. Observe the shape of the bubbles which
are in contact each other. With a microscope, observe a thin section of elder pith and
compare it with the foam.

2 - Make a solid foam. Beat egg whites and some sugar, then cook it so to
obtain its solidification: you will have obtained a meringue, just an edible solid foam.

1 - Who can guess more colloids? List the colloids you have in your
home or which you know by experience: (milk, mayonnaise, resin, paint, ink, expanded
polystyrene, cell cytoplasm, blood serum, etc.).

2 - A half-solid fluid. Put in a cup four spoons of corn starch. Add
some water until you have obtained a creamy substance. While mixing, you will notice that
this substance has an odd property: if you slowly mix it, it behaves like a liquid, but if
you try to mix it fast, it seems solid. By quickly lifting it on a side, you will be also
able to remove this cream from the cup, but you will have some difficulties in keeping it
in your hands because, even if it moves slowly, it will escape from all sides like a
liquid. Liquids which change viscosity with the mixing speed are called dilatant
fluids. Also wet sand behaves as dilatant fluid. Sold in the US as a childs toy
under the name of Gak or Goo, you can make your own by dissolving 1/2 cup of white glue
with 1/2 cup of water, then adding 3 tablespoons of Borax, while stirring well. You will
obtain a substance which is apparently solid, but which loses its shape within some
minutes, becoming like a liquid puddle... which however you will able to lift it as if it
was a carpet.

ATOMIZER FOR AEROSOLHow do atomizers work? There are many models of atomizers or of sprayers like those of
pressurized spray paint cans, or those provided with a small pump that you press with a
finger, those that work by mean of a rubber syringe or, for industrial uses, by a
compressor.

1 - Anatomy of an atomizer.

Disassemble a
trigger spray bottle. Often, these devices breaks so, if you have one of them broken,
dismantle it to try to understand why it does not work any more and try to repair it.http://www.howstuffworks.com/question673.htm
Trigger spray bottle.

2 - Build an atomizer. To build a small
atomizer, take two thin straws and fix them as shown in figure 30. At the end of the
horizontal straw, insert a plug with a hole of one mm of diameter. Under the vertical
straw, mount a small bottle with water. Now, blow with force in the horizontal straw. The
air jet which comes out of the hole will cause an area of low pressure above the vertical
can which will draw some water up the straw and blow it away atomizing it. To produce an
air jet, you can also use a rubber syringe. Usually, this type of atomizers is used for
perfumes, but you can use it also to humidify the leaves of a house plant.

Surface phenomena and colloids concern many objects, products and events
of our everyday life, which are not immediately explainable with the physics we usually
study at school. Having introduced some principles, and suggested some experiments in this
field which until now may have been quite mysterious seemed to us useful and important.
Not only, but you have also noticed how fascinating these topics are and how amusing is to
do laboratory activities with them.